Are Carbs Still King? Fuel System Technologies is Betting Yes

A popular Mark Twain misquote has him offering, “The rumor of my death has been greatly exaggerated.” The reality is his quote was greatly inflated, but we’ll use it for our purposes to make the point that the rumors of the demise of the carburetor have also been subjected to significant embellishment.

Even the simplest of the Fuel Systems Technology (FST) offerings like this 650-cfm RT vacuum-secondary version uses what FST calls a pressure-based casting that offers an exceptionally clean appearance. This version is a single-inlet carb with a metering plate instead of the more performance-oriented metering block with changeable jets.

To reinforce that concept, we give you Fuel Systems Technology (FST): a brand-new carburetor company out of the Detroit area that offers some very nicely built, American-made fuel mixers that offer some interesting fuel-metering opportunities. This is not new technology, but rather an enhancement of off-the-shelf carburetion with an eye toward making the mixing of air and fuel a little easier.

The Billet X-treme is intended for the upscale enthusiast looking for pure race adjustability. This mechanical-secondary 750 unit features multiple opportunities. The main body, float bowls, metering blocks are billet aluminum. Note how the throttle plate offers a dual pattern to bolt to standard as well as 4500-style mounting flanges.

Let’s start with some basics. The FST line consists of eight different levels of sophistication from the RT line of cast main body carburetors that extend through the RTX Viper line that are enhanced with the Billet series that offers billet main bodies, metering blocks, and fuel bowls in multiple configurations. The cfm lineup spans from an entry-level, vacuum-secondary, 600-cfm unit all the way up to FST’s Billet Excess Pro line that lives up to its name by flowing as much as 1,450 cfm.

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We disassembled this FST Billet X-treme 750 to reveal the primary metering block and float bowl, and its conventional configuration for the metering block, power valve, and main jets.

If appearances are worth anything, even the entry-level pieces are very nice, but our approach here is more concerned with their functionality. That said, even the smallest, single-inlet carb offers screw-in, air-bleed adjustments that are normally only found on more upscale and expensive carburetors. While basic metering on all these carburetors should be very close right out of the box, many enthusiasts think there is no advantage to tuning. But even slight air-bleed adjustments can offer noticeable driveability improvements, so the ability to make these changes easily with screw-in bleeds is a nice feature.

This tight view of the Billet X-treme reveals an interesting option that can include a throttle position sensor. This makes it easy to convert to an electronic-feedback transmission like a 4L65E or 4L80E automatic. Also note the four-corner idle capability.

As you can see from the images, this entire lineup is based on the original standard-flange Holley design and offers many of the same advantages. Among the more interesting aspects that would appeal to the knowledgeable carb tuner beyond the screw-in air bleeds is the option beginning with the Billet carburetors of an idle bypass feature. Again, this is not new technology, but FST’s approach is slightly different. With the billet main body carburetors, the air-cleaner stud is surrounded by four small screw-in fittings that come from the factory blank (undrilled). These plugs allow access to intake manifold vacuum. By drilling small holes in these fittings or removing them altogether, it’s possible to custom-adjust the idle speed for engines with very large camshafts that idle at vacuum levels below 10 inches of manifold vacuum.

We ran a few of FST’s carburetors on the dyno in a short tuning session and were impressed with how easily these carbs respond to even minor tuning changes. FST’s Phil Freland demonstrated just how quickly he could make a significant change to the entire fuel curve with a couple of minor emulsion-hole changes.

The tuning issue this addresses is that big-cam engines require much more throttle opening to create a proper idle speed. This requires a large primary throttle blade opening that uncovers the idle transfer slot. This delivers additional idle fuel not metered by the idle mixture screws, making the engine run very rich and less responsive to the idle mixture adjustments. Bypassing air using the FST bleeds allows the tuner to close the throttle blade position back to the proper position with the idle transfer slot barely uncovered. This allows far more control over idle mixture and eliminates the classic off-idle stumble so common with big-cam, carbureted engines. These air-bypass fittings eliminate the need to drill air-bypass holes in the throttle blades, which was previously the only way to address this problem.

An example of the ability to tune is this tight shot of an FST metering block with the four emulsion holes. By changing the size of one or more of the emulsion holes, a tuner can make changes to the shape of the fuel curve.

Four-corner idle is also available on nearly every FST model (except a couple of the entry-level RT single-inlet carburetors), which is also a plus for engines with bigger camshafts that can benefit from a more even idle mixture pattern in the intake manifold. Another interesting option available on many of the FST carburetors is an option for what is called the RTX Viper float bowl that creates a center inlet for the billet fuel bowl. At first, this seemed of minor consequence until we attended the 2018 SEMA Show, where FST introduced its electronic-feedback system that incorporates an electronic solenoid at this center fuel inlet. The solenoid is controlled by a small ECU that receives air/fuel mixture feedback from a wideband O2 sensor located in the exhaust. This electronic option will be offered on complete carburetors or as an add-on to an existing FST carburetor. FST tells us this package is as yet not ready for production, but should be on shelves by the second half of 2019.

On the large-by-huge flow side of things, FST also has that area covered with monster cfm capacity that ranges up to 1,450 cfm. This 1,250-cfm Billet Excess carb has all the cool options, including a billet-aluminum fuel log. This happens to be a four-circuit carb intended for the most serious of tuning ventures.

But even without the feedback idea, there are other FST tuning opportunities that might attract the more sophisticated carb aficionado. Most carburetors operate as a two-circuit, fuel-delivery system. This refers to the number of bleeds located at the top of each venturi. A two-circuit carb uses one air bleed for idle and the other for main fuel delivery. FST offers options that will include both three- and four-circuit systems that can be quickly identified by the number of bleed holes at the top of each barrel. The three- circuit concept first came into play years ago on the Dominator-style race carburetors, and FST has expanded this option to a wider selection of sizes.

This view of the billet-aluminum float bowl reveals the two small quick-change jet hole slots that offer the ability to change jets without removing the float bowl. This requires the use of a jet holder tool that captures the jet during removal and installation.

These additional circuits are mainly used for additional midrange and top-end tuning that cannot be accomplished merely by changing jets. These are not really intended to be used on the street, but intended more for applications like throttle-stop drag racing, where the tuner can more accurately create the ideal fuel curve throughout the entire rpm band.

This Billet X-treme 750 carb shows the four small idle bypass plugs. These bypass air directly to the intake manifold. By drilling or removing them, it’s possible to quickly establish the proper idle speed without disturbing the curb idle setting on the primary throttle blades.

To create tuning opportunities with a given carburetor, you used to purchase custom parts and make the conversion yourself. What FST has essentially done is offer many of these options now as standard equipment. It’s a little bit like the car industry: What was once considered a luxury option is now standard fare. The best part? The price is right for this new line, and there are dozens of options; to dial in exactly what you might need, visit the FSTcarb.com website and poke around with all of those available options.

The typical carburetor uses a two-circuit bleed system, but FST offers an optional three- (shown here) or four-circuit possibility. In this three-circuit system, for example, the third bleed adjusts midrange fuel delivery.

When it comes to simple induction options, it doesn’t get much cleaner or simpler than a 750- or 850-cfm carburetor on top of an aluminum intake manifold. Now that tuning knowledge is expanding, it’s more and more likely that with a little bit of effort, you can deliver near-EFI-like performance from a fuel-metering device like a carburetor. There’s still quite a future ahead for these simple devices, especially if you’re hip to making them work to their full ability.